The present invention relates generally to sodium-sulfur batteries and more particularly to the utilization of a specific header, preferably a metal header, in this battery and a particular method of assembling the header.
A conventional sodium-sulfur battery of the prior art typically includes a first metal container which contains sulfur in a molten state, a second metal container which contains sodium in a molten state, and a beta-alumina tube extending into the sulfur in the first container but which opens to the second container for receiving the molten sodium therein. In this overall configuration, the first container, that is, the container including molten sulfur, acts as the positive electrode of the battery; the second container, that is, the container including molten sodium, acts as its negative electrode; and the beta-alumina tube acts as the solid electrolyte.
From the foregoing, it should be apparent that the two containers must be physically isolated and electrically insulated from one another. In a conventional sodium-sulfur battery design, this is typically accomplished by means of a ceramic header, specifically a header constructed of alpha-alumina. This header is located between the two containers and around the beta-alumina tube. While the juncture between the header and each container can be readily sealed by means of a mechanical bond or other nonmetal-to-metal bond, it is extremely difficult to provide a reliable seal directly between the header and the electrolyte tube. This is primarily because it is difficult to directly seal alpha-alumina and beta-alumina to one another. As a result, it has been necessary heretofore to utilize an intermediate component, typically a sodium resistant sealing glass, to join the two. There are however a number of particular drawbacks to this approach. First, it has been found that the sealing glass reacts with the beta-alumina to weaken the tube. Specifically, it has been found that during the forming process the glass has a tendency to draw out some of the sodium ions from the beta-alumina causing the formation of alpha-alumina crystals within the tube. Second, it has been found that over a period of time there is a degradation of the sealing glass and header materials caused by a chemical reaction with the molten cell reactants, specifically with the molten sodium. Third, there tends to be a devitrification of the sealing glass. Fourth, the sealing glass must have a coefficient of expansion compatible with the alpha and beta alumina.
As will be seen hereinafter, the sodium-sulfur battery of the present invention and its method of manufacture eliminate the foregoing disadvantages. Moreover, as will also be seen, this is accomplished in a reliable and yet uncomplicated and economical way.